1. Field of the Invention
The present invention relates to a production method of cellulose film in which cellulose film is produced by preparing a polymer solution through dissolving cellulose ester in a solvent containing a prescribed organic solvent as the main component, forming a filmy object from the prepared polymer solution, and evaporating the solvent in the filmy object, and a cellulose film thus produced; and protective film for polarizing plate, optical functional film, polarizing plate, and liquid crystal displays produced by the above described production method of cellulose film.
2. Description of the Related Art
Conventionally, cellulose film is used as optical materials for optical functional films for use in widening the viewing angle and preventing glare; protective films for the polarizing plates in liquid crystal displays; and the like. The cellulose films used for such optical materials are produced by means of the solution method for forming film. In the solution method for forming film, a filmy object is formed from a polymer solution in which cellulose ester or the like is dissolved in an organic solvent, the filmy object formed is heated to evaporate the organic solvent in the filmy object, and a polymer film is thereby obtained. In this connection, when the organic solvent remains in the produced cellulose film, there occur adverse effects on the dimensional stability of the film, or the coloring of the film is degraded. Accordingly, the control of the residual amount of the organic solvent in the produced cellulose film has heretofore been performed from the viewpoint of the quality. The produced cellulose film is subjected to saponification and the like in post-processes, and subsequently is commercialized as optical functional film, or protective film for polarizing plate.
Now, when the cellulose film produced by the solution method for forming film is considered from the standpoint of the environment conservation, nowadays considered to be important, there is concern that, with the level of the residual amount of the organic solvent as controlled from the viewpoint of the quality, a slight amount of the organic solvent is evaporated from the produced film in the post-processes subsequent to the film production.
However, as for the residual amount of the organic solvent, investigation has hitherto been performed from the viewpoint of the quality, but no investigation has been performed from the viewpoint of the environment conservation, and hence it is not clear how far the level of the residual amount of the organic solvent should be lowered so that the effects on the environment substantially vanish.
Additionally, in order to reduce the residual amount of the organic solvent, the following treatments are suggested: the heating period of time is extended in the evaporation process, the heating temperature is raised, and the amount of the organic solvent is reduced in relation to the amount of cellulose ester. However, in the current solution method for forming film, the production efficiency is improved while the quality of the film being maintained at a high level, and hence a variety of measures are adopted for shortening the time required for the evaporation process as much as possible; these measures include the following measures in which the amount of the organic solvent is decreased to a level as low as possible in relation to the amount of cellulose ester, and the heating temperature is raised to a level below which cellulose ester is not thermally decomposed. Consequently, it is anticipated that not only the quality of the film is degraded, but also the production efficiency is remarkably degraded, owing to extending of the heating time in the evaporation process, raising the heating temperature, and reducing the amount of the organic solvent in relation to the amount of cellulose ester, for the purpose of reducing the residual amount of the organic solvent.